Recently, ultra-short pulse light having short pulse length, such as subpicosecond pulse length, becomes necessary with increasing demands for high-speed optical communications. Meanwhile, the ultra-short pulse light has become difficult to generate due to limits on the high speed of electric circuitry. In view of this, there is developed an ultra-short pulse light source which utilizes wavelength dispersion and nonlinearity of an optical fiber to generate ultra-short pulse light with use of an optical compressor for compressing pulse length. FIG. 11 conceptually illustrates a structure of the above-mentioned ultra-short pulse light source.
In FIG. 11, an ultra-short pulse light source 1 is structured to have, for example, an optical pulse source 11 for emitting pulse light having several-picosecond pulse length, an optical amplifier 12, such as an EDFA (Er-Doped Fiber Amplifier), for amplifying pulse light emitted from the optical pulse source 11, and a compressor 13, composed of an optical fiber or the like, for compressing the optically-amplified pulse light.
FIG. 12 illustrates an example of an optical fiber used in the compressor 13. The optical fiber which constitutes the compressor 13 includes an adiabatic soliton compressor which utilizes wavelength dispersion and nonlinearity of the optical fiber, such as DDF (Dispersion-Decreasing Fiber, see non-patent document 1, for example) having the wavelength dispersion decreasing in the longitudinal direction of the fiber, SDPF (Step-like Dispersion Profiled Fiber, see non-patent document 2, for example) in which a plurality of optical fibers having different dispersions is used changing the dispersion profile in the longitudinal direction of the fiber, CDPF (Comb-like Dispersion Profiled Fiber, see non-patent document 3, for example) in which two optical fibers having different dispersions are used changing the dispersion profile in the longitudinal direction of the fiber. The adiabatic soliton compression is a method for compressing optical pulses with use of effects of nonlinearity and dispersion, imparting to the basic soliton a gentle perturbation inside the fiber, and is advantageous in that there is so little degradation in time waveforms or spectral profiles. In the DDF method, it is possible to gently decrease the dispersion so that ideal adiabatic soliton compression can be achieved. However, it is difficult to manufacture an optical fiber in which dispersion parameters decrease in an ideal manner along the length of the optical fiber. Accordingly, the SDPF and CDPF methods have been proposed, using a combination of a plurality of optical fibers of different dispersions to decrease an average dispersion along the length of the fiber thereby to achieve a DDF profile by approximation. However, from the standpoint of compression efficiency and ease of manufacture, it is preferable to use CPF (Comb-like Profiled Fiber, see non-patent document 4, for example) which utilizes HNLF as nonlinear medium and dispersion of SMF (Single Mode Fiber) as dispersion medium.
In the above-described ultra-short pulse light source 1, usually, it is difficult to obtain linearly-polarized light outputs because the polarization direction is apt to vary in the optical fiber. Then, in order to maintain the polarization direction, there is disclosed a method of manufacturing a compressor 13 with an optical fiber having a polarization maintaining material (for example, see non-patent document 1). Meanwhile, as shown in FIG. 13, there is disclosed a technique for using a mirror Faraday rotator in a mode locked ultra-short pulse oscillator to compensate variation in the polarization so that one polarization state is maintained during operation (for example, see non-patent document 5).
In FIG. 13, an ultra-short pulse light source 2 has a pumping light source 21, a WDM (Wavelength Divisional Multiplexer) 22, an EDF 30, a polarizer 24, a wavelength plate 25, a mirror Faraday rotator 28 (Faraday rotator with mirror), a focusing lens 27, a Faraday rotator 26 and a mirror 29. The pumping light source 21 pumps the EDF 30 via the WDM 22. The mirror Faraday rotator 28 outputs light having the polarization rotated 90 degrees with respect to that of the incident light. Then, the light travels from the Faraday rotator 26 to the mirror Faraday rotator 28 and vice versa, so that variation in polarization in the non polarization maintaining fiber can be compensated. With this structure, it is possible to obtain light having linear polarization perpendicular to that of the linearly-polarized light input to the mirror Faraday rotator 28.
[Non-Patent Document 1]
K. R. Tamura and M. Nakazawa: “54-fs, 10-GHz soliton generation from a polarization-maintaining dispersion-flattened dispersion-decreasing fiber pulse compressor,” Optical Letter, Vol. 26, p. 762, 2001)
[Non-Patent Document 2]
S. V. Chernikov et al.: “Experimental demonstration of step-like dispersion profiling in optical fiber for soliton pulse generation and compression,” Electron. Letter, Vol. 30, p. 433, 1994
[Non-Patent Document 3]
S. V. Chernikov et al.: “Comblike dispersion-profiled fiber for soliton pulse train generation”, Opt. Lett., Vol. 19, p. 539, 1994
[Non-Patent Document 4]
K. Igarashi et al.: “Ultra-highly pure 160 GHz subpicosecond soliton train generation with average dispersion-managed comb-like dispersion profiled fiber using highly-nonlinear fiber,” CLEO2003, CMH 7 page, 2003
[Non-Patent Document 5]
M. E. Fermann et al., “Environmentally Stable Kerr-type mode-lock erbium fiber laser producing 360-fs pulse”, Optical Letter, Vol. 19, p. 43, 1994